1. Field of the Invention
The present invention relates to a display device comprising a light-emitting layer such as an electroluminescence (hereinafter referred to as xe2x80x9cELxe2x80x9d) element disposed on a substrate, and particularly to a sealing structure for an organic EL display device.
2. Description of the Prior Art
In the recent years, organic EL display devices have gained attention as devices that may replace CRTs and LCDs. FIG. 1A is a plan view of a conventional organic EL display device, and FIG. 1B shows a cross-sectional view taken along line A-Axe2x80x2 of FIG. 1A. A plurality of selective drive circuits 2 are disposed for respective pixels on a transparent substrate 1. A planarizing insulating film 3 is formed covering the selective drive circuits 2. A contact hole is created in the planarizing insulating film 3 in a position corresponding to each selective drive circuit 2. Each selective drive circuit 2 is connected to a pixel electrode 4 through this contact hole. An emissive layer 5 and a counter electrode 6 are disposed covering those structures. Surrounding the pixel region including the selective drive circuits 2, pixel electrodes 4, emissive layer 5, and counter electrode 6, display driver circuits 7a, 7b are arranged for controlling the selective drive circuits 2 and applying predetermined voltages to the pixel electrodes 4 so as to drive the display region. The driver circuits 7 are connected to terminals 9 by wiring 8. A protective casing 10 composed of a metal such as aluminum is arranged covering these structures, and adhered to the transparent substrate 1 using an adhesive 11 made of a resin cured by ultraviolet rays. The sealed space between the protective casing 10 and the transparent substrate 1 is filled with dry nitrogen. A desiccant sheet 12 is disposed on the inner surface of the protective casing 10.
A selective drive circuit 2 comprises, for example, a plurality of semiconductor elements including thin film transistors (TFT). A first TFT switches between xe2x80x9conxe2x80x9d (conductive state) and xe2x80x9coffxe2x80x9d (non-conductive state) in response to the output from the driver circuit 7a. When the first TFT of a selective drive circuit 2 is turned on by an output from the driver circuit 7a, the corresponding pixel electrode 4 is applied with a voltage according to an output from the driver circuit 7b via a second TFT. An electric current thereby flows between the pixel electrode 4 and the counter electrode 6. The emissive layer 5 is configured to emit light when a current is made to flow therein by the pixel electrode 4 and the counter electrode 6, and emits light at an intensity according to the amount of current flowing between the pixel electrode 4 and the counter electrode 6. The generated light transmits downward in the cross-sectional view through the transparent substrate 1 to be observed.
In the organic EL element, holes injected from the anode and electrons injected from the cathode recombine within the emissive layer. As a result, organic molecules constituting the emissive layer are excited, generating excitons. Through the process in which these excitons undergo radiation until deactivation, light is emitted from emissive layer. This light radiates outward through the side of the transparent anode via the transparent insulator substrate, resulting in light emission.
An organic EL layer 5 is known to be susceptible to degradation by moisture. When, for example, a defect such as a pinhole is present in the counter electrode 6, moisture entering from the pinhole may cause oxidation of the counter electrode 6 or separation between the organic EL layer 5 and the counter electrode 6, producing dark spots and resulting in deterioration of display quality. It is the function of the protective casing 10 not only to protect the display region and the driver circuits 7 from physical shock, but also to prevent moisture from entering the device. The protective casing 10 is therefore formed in a shape of a tray covering the display region. Further, to prevent damage by penetrating moisture, the space inside the protective casing 10 is filled with an inert gas such as dry nitrogen or helium, and the desiccant sheet 12 is disposed. A stepped portion may be provided in the location for arranging the desiccant sheet 12. The structure as described above is disclosed, for example, in Japanese Patent Laid-Open Publication No. Hei 9-148066.
However, in a conventional sealing structure, the protective casing 10 is adhered to the transparent substrate 1 by directly applying the adhesive 11 on the transparent substrate 1. By such a method, the adhesive may peel off during curing of the adhesive 11 due to the difference in the coefficient of thermal expansion between the transparent substrate 1 and the protective casing 10, resulting in incomplete sealing.
Moreover, the adhesive 11 is applied over the wiring 8 in areas provided with the wiring 8. A disconnection may be caused in the wiring due to stress generated during curing of the adhesive 11.
The object of the present invention is to provide an EL display device having a structure preventing separation between the transparent substrate 1 and the protective casing 10 even when there exists a difference in the coefficient of thermal expansion between the transparent substrate 1 and the protective casing 10, while also preventing disconnection in the wiring 8.
The present invention for achieving the above object provides a display device having a display region arranged between first and second substrates composed of different materials, comprising a seal for adhering the first and second substrates to one another, and a buffer layer between the seal and the first and/or second substrate.
The buffer layer may be an insulating film.
In another aspect of the present invention, the display region is configured by laminating a plurality of thin films including an insulating film. The insulating film extends to an area between the seal and the first and/or second substrate to serve as the buffer layer.
In a further aspect, the display region comprises selective drive circuits provided for each pixel, a planarizing insulating film formed covering the selective drive circuits, and pixel electrodes disposed on the planarizing insulating film corresponding to each of the selective drive circuits. The planarizing insulating film serves as the buffer layer.
The first substrate may be a transparent insulating substrate, and the second substrate may be a protective casing formed covering the display region.
Further, the first substrate may be made of glass or resin, and the second substrate may be made of metal.
In another aspect of the present invention, the planarizing insulating film extends in an area between the first substrate and the seal.
The planarizing insulating film may be composed of a material softer compared to the seal and the first substrate.
In a further aspect of the present invention, the display device includes a terminal connected to the display region via a wiring and exposed outside the display device. The buffer layer is arranged between the wiring or the terminal and the seal.
Another aspect of the present invention is a light-emitting device in which an emissive region having an emissive element is sealed between first and second substrates having different coefficients of thermal expansion, wherein the first and second substrates are adhered to one another by a seal in an area surrounding the emissive region, and a buffer layer is formed between the seal and the first and/or second substrate.
A further aspect of the above-described device of the present invention is that a desiccant is mixed in the seal.
According to the present invention, the element provided in the display region or the emissive region may be an organic electroluminescence element containing an organic emissive material.
In a further aspect of the present invention, the emissive region comprises selective drive circuits provided for each pixel, a planarizing insulating film formed covering the selective drive circuits, and pixel electrodes disposed on the planarizing insulating film corresponding to each of the selective drive circuits. The planarizing insulating film serves as the buffer layer.
By providing a buffer layer as described above, stress generated at the seal portion due to causes such as the difference in the coefficient of thermal expansion between the first and second substrates can be absorbed by the buffer layer in the present invention. Accordingly, defects in adhesion between the first and second substrates due to peeling of the seal or other reasons can reliably be prevented.
According to the present invention, the display region is configured by laminating a plurality of thin films including an insulating film, and the insulating film serves as the buffer layer. It is therefore unnecessary to separately provide a buffer layer. The buffer layer can be formed at the same time of forming the display region, simplifying the manufacturing process.
In the present invention, the planarizing insulating film serves as the buffer layer. A planarizing insulating film has a greater thickness compared to a gate insulating film or an interlayer insulating film, and is softer than glass or the seal. Among the insulating films formed within the display region, the planarizing insulating film is most suitable as the buffer layer.
Further, according to the present invention, the display device includes a terminal connected to the display region via a wiring and exposed outside the display device, and a buffer layer is arranged between the wiring or the terminal and the seal. Such an arrangement prevents disconnection of the wiring caused by the stress generated during curing of the seal.